This invention relates to a method of implementing intercommunication systems for wireless transmission of speech with the aid of asynchronous or synchronous ternary delta modulation.
An intercommunication system enables several subscribers to exchange information simultaneously via a common signal channel. The signal channel used is frequently a line. In the line, the audio frequency signals of the participants in the conversation are additively superimposed upon one another.
In order to preserve the mobility of the subscribers, wireless-intercommunication systems have to be used. Due to the interference of the high-frequency carriers, the additive superposition of the audio frequency signals in the signal channel is impractical with the customary modulation methods. In radio telephony, therefore, the simplex method is used in which it must be ensured that only one subscriber is transmitting in each case and the other subscribers are only able to receive at this time.
The output signal of an asynchronous delta modulator is a ternary signal of which the three amplitude values of -1, 0, +1 are quantized, but not the repetition frequency. The frequency of occurrence of a +1 or a -1 signal corresponds to the steepness of the edge of the low-frequency signal applied. A more detailed discussion of asynchronous ternary delta modulation may be found in the book "Delta Modulation Systems" by R. Steele, Pentech Press, London, 1975, ISBN 0727304011, pp. 165-182 and in "Electronic Engineering", Vol. 40, 1968, pp. 32-37. With the probability distributions of the amplitudes and spectrum of human speech the signal probability p.sub.s is only a few percent for the occurrence of a +1 or -1 signal, depending on the design of the delta modulator and the pitch of the speaking person.
For some applications it is also desirable if such modulators work synchronously. If the decision circuit and the clock-controlled memory of a customary binary synchronous delta modulator are replaced by ternary systems, a ternary synchronous delta modulator is obtained, the output signal of which is similar to that of the asynchronous delta modulator but the output signal of which is synchronized with the clock signal. This represents a special case of the modulator known under the name of "Multilevel Delta Modulator" as described by R. Steele in "Delta Modulation Systems", Pentech Press, London, 1975, pp. 301-323.